A read channel circuit in a magnetic disk drive device includes components for processing the analog read signal generated by the read/write head of the device. This processing provides automatic gain control (AGC) amplification, filtering, and equalization, as well as analog-to-digital conversion.
A hard drive can read data by detecting a voltage peak that is sensed when a magnetic flux reversal on a magnetic disk passes underneath the read/write head. More recently, a partial response maximum likelihood (PRML) algorithm has been introduced to interpret the magnetic signals sensed by the read/write heads. PRML disk drives read the analog waveforms generated by the magnetic flux reversals stored on the disk. Rather than look for peak values to indicate flux reversals, PRML digitally samples the analog waveform (the “partial response” portion of the algorithm) and applies signal processing to determine the bit pattern represented by the waveform (the “maximum likelihood” portion of the algorithm).
In any case, a normalized readback signal amplitude is often required for proper data detection. To this end, a variable gain amplifier (VGA) may be used in the analog signal path for scaling the readback signal. Because of material and manufacturing variations, each head has a different characteristic signal output level than other heads.
First, the VGA is calibrated using a square wave generator such as a digital to analog converter (DAC). Once the VGA is calibrated, a test signal from the read head is sent. The resulting VGA gain code is recorded. From this process, the input signal from the read head can be characterized for future use in actual data read operations.
It can readily be recognized that it is important to know the input signal to the VGA during calibration, and one way to do this is to input known test signals. However, this requires a very accurate test signal source. As recognized herein, it is desirable to undertake the above read head signal characterization process without requiring tight tolerances of the test signal source.